O R I G I N A L P A P E R
Substituent Effect on the Reactivity of Alkylated Triphenyl
Phosphorothionates in Oil Solution in the Presence of Iron
Particles
Filippo Mangolini•Antonella Rossi •
Nicholas D. Spencer
Received: 29 May 2010 / Accepted: 7 July 2010 / Published online: 30 July 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The effect of the substituent attached to the phenyl rings on the reactivity of alkylated triphenyl phosphorothionates (t-butyl TPPT (b-TPPT) and p-nonyl TPPT (n-TPPT)) in oil solution at high temperature (423 and 473 K) was investigated by means of Fourier-trans-form infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The FT-IR and NMR results show that the alkylated TPPTs were highly thermally stable and did not completely decompose in oil, even upon heating at 423 K for 168 h and at 473 K for 72 h, with and without steel filings and iron particles (both metallic iron and iron oxide particles). The reaction of alkylated TPPTs was found to start with the scission of the P=S bond to yield alkylated triphenyl phosphate. The kinetics of the thermo-oxidative reaction was slower when steel filings and iron particles were added to the oil solutions during the heating experiments. The reactivity of the unsubstituted molecule (TPPT) was higher than that of alkylated TPPTs at 423 K, while at 473 K TPPT and n-TPPT were more reactive than b-TPPT. In the case of the experiments performed at 473 K in the presence of steel filings or metallic iron or iron oxide particles, the reactivity of the alkylated TPPT molecules decreased with the length of the alkyl chain bound to the
phenyl rings. The XPS results show that a reaction layer consisting of carbon, oxygen, phosphorus and iron was formed on the 100Cr6 steel filings immersed for 72 h in oil solutions containing alkylated TPPTs and heated at 473 K. Sulphur was neither detected on the surface nor in the composition vs depth profile. During the heating experi-ments, the base oil (PAO) was oxidized. At 423 K, the alkylated TPPTs had a strong antioxidant effect, which was found to be more pronounced upon increasing the length of the alkyl chain bound to the phenyl rings. At 473 K, the TPPTs did not inhibit the oxidation of the base oil as effectively as at 423 K.
Keywords Thermo-oxidative degradation t-butyl triphenyl phosphorothionate (b-TPPT) IrgalubeÒ232 p-nonyl triphenyl phosphorothionate (n-TPPT)
IrgalubeÒ211 Ashless anti-wear additives Low-SAPS additives FT-IR NMR XPS Iron particles
1 Introduction
The development of exhaust after-treatment systems for gasoline and diesel automobile engines over the last 20 years has brought about a need to change the engine-lubricant composition, in order to minimize the degrada-tion of catalysts and filters over the operating life of an engine. Three main components of engine lubricant addi-tives, i.e. phosphorus, sulphur and metals, have been demonstrated to lead to a loss of emission-control catalyst effectiveness and to block filters within the exhaust after-treatment system [1]. Consequently, modern engine-lubricant specifications are progressively limiting the permissible levels of sulphated ash, phosphorus and sul-phur (SAPS) in oil formulations [2].
F. Mangolini A. Rossi N. D. Spencer (&)
Department of Materials, ETH Zurich, Laboratory for Surface Science and Technology, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland
e-mail: nspencer@ethz.ch A. Rossi
Dipartimento di Chimica Inorganica ed Analitica, Universita` degli Studi di Cagliari, INSTM unit - Cittadella Universitaria di Monserrato, 09100 Cagliari, Italy
Among the many additives that are employed for for-mulating engine lubricants, zinc dialkyldithiophosphates (ZnDTP) probably represent one of the most successful class of compounds ever synthesized. First introduced as antioxidants in the 1940s (they act as both primary (i.e. radical trapping) and secondary (i.e. peroxide-decompos-ing) antioxidants), it soon became clear that ZnDTPs also combine good antiwear and extreme-pressure properties [2–9]. The performance of ZnDTPs is strongly influenced by the chemical structure of the molecule, which mainly depends on the type of alcohols used for the synthesis [8]. Moving from primary alkyl to secondary alkyl ZnDTPs, the oxidative inhibition and wear protection performance improves, while the thermal stability decreases. This structure-activity dependence of ZnDTPs was explained by the ease with which the nucleophilic substitution at the a-carbon atom of the alkyl group can take place [10]. Compared to the alkyl varieties, aryl ZnDTPs usually show inferior antioxidant and antiwear performance, but superior thermal stability [8].
In spite of the remarkable effectiveness of ZnDTPs over a wide range of conditions, the large amounts of all three catalyst-poisoning agents (phosphorus, sulphur and zinc) present in this class of compounds brought environmental considerations of their use in engine oils into focus. Although the concentration of ZnDTPs in lubricant for-mulation has already been reduced [11], the development of new additives with lower- or zero-SAPS levels is becoming a necessity [12,13].
Among the different chemistries that have been pro-posed as replacement or supplement to ZnDTPs in engine oils, ashless equivalents to ZnDTPs, i.e. thiophosphates and phosphates without any metal, have already been investi-gated quite extensively. In the last decade, the antiwear performance as well as the nature and properties of the films formed on tribological surfaces by these new envi-ronmentally friendly additives have been studied by several research groups [12].
Hilgetag and Teichmann [14,15] described the chemical behaviour of pure organophosphates and organothiophos-phates, focusing, in particular, on the difference in the nucleophilic reactivities of the phosphoryl and thiophos-phoryl groups. The key chemical feature of these classes of compounds is their strong alkylating power. All the reac-tions were explained on the basis of Pearson’s acid–base concept [16–19], according to which oxygen is a ‘‘hard base’’ and preferentially reacts with ‘‘hard acids’’, such as protons, ‘‘A’’ metals (alkali metal cations, transition metal cations in high oxidation states), carbonyl carbon and phosphoryl phosphorus. In contrast, sulphur is a ‘‘soft base’’ and mainly reacts with ‘‘soft acids’’, such as ‘‘B’’ metals (transition metal cations in low oxidation states), tetrahedral carbon and halogens.
The reactivity of tributyl thiophosphate (TBT) with air-oxidized iron surfaces at 373 K has been investigated by in situ attenuated total reflection (ATR/FT-IR) tribometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction spectroscopy (TPRS) [20]. Thermal films and tribofilms were found to consist of iron poly-phosphate and sulphate. Compared to the reaction layers generated under purely thermal conditions, the amount of sulphate and polyphosphate was higher in the films formed under tribological conditions in the boundary-lubrication regime. The comparison of the surface chemistry of TBT with that of tributyl phosphate (TBP) allowed the authors to clarify the role of sulphur in this class of molecules: it facilitates the formation of long-chain polyphosphates and lowers the temperature of the chemical and tribochemical reactions by around 50 K.
The study of the chemical reactions occurring in oil at high temperature could provide an insight into the mech-anism of thermal film and tribofilm formation as well as into the antioxidant properties of an additive, as demon-strated in the case of ZnDTPs [4,6]. For this reason, the reactivity of triphenyl phosphorothionate (TPPT) in syn-thetic oil (poly-a-olefin, PAO) at 423 and 473 K has been recently investigated in our group by Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic res-onance (NMR) spectroscopy [21]. The FT-IR and NMR results showed that this molecule possesses high thermal stability in lubricant oil solution. A thermo-oxidative reaction was found to start with the scission of the P=S bond and to result in the formation of triphenyl phosphate (TPP). The oxidation of the base oil, which involves a four-step free-radical reaction (initiation of the radical chain reaction, chain propagation, chain branching and termination of the radical chain reaction), leading to the formation of a complex mixture of oxygenated products (e.g. hydroperoxides, alkyl peroxides, alcohols, carboxylic acids, peroxy acids, esters, ketones, aldehydes and lac-tones) [8,22,23], also suggested that the TPPT molecule is not an effective oxidation inhibitor, in contrast to ZnDTPs.
The presence of metallic or oxidized iron particles in the oil solution during the heating experiments was shown to catalyze the reaction of TPPT to yield TPP, but not to change the reaction mechanism [24]. However, the con-centration of TPP in oil solution generated by the thermo-oxidative reaction of TPPT was lower when the heating experiments were carried out in the presence of metallic and oxidized iron particles. The XPS results showed that a reaction layer was formed on the 100Cr6 steel filings immersed for 72 h in TPPT solution heated at 473 K and was found to consist of carbon, oxygen, phosphorus and iron. The XPS sputtering depth profile revealed the pres-ence of sulphur in the reaction layer, starting from a
sputtering depth of 3.5 ± 0.1 nm relative to a Si/SiO2
reference sample.
The surface reactivity of alkylated phosphorothionates (triphenyl phosphorothionates (TPPTs) substituted with alkyl chains of different lengths) with air-oxidized steel was also investigated in our group [25, 26]. The activation temperature for the thermal decomposition of TPPTs was found to be around 423 K. At 423 K a thin reaction layer, consisting of short-chain polyphosphates and oxidized sul-phur species, was formed in the non-contact region of air-oxidized steel (thickness of the oxide layer: 2.7 ± 0.2 nm) after ca. 5 h, as indicated by the XPS results. The compo-sition of the layer generated at 423 K was found to be dependent on the length of the alkyl chains bound to the phenyl rings: with longer chains, the phosphate concentra-tion decreased, while the sulphur concentraconcentra-tion increased. On the basis of these findings, a reaction mechanism was proposed: following the adsorption of the sulphur atom to the surface, as suggested by Koyama et al. [27], the phosp-horothionate molecules started decomposing with the scis-sion of the P=S bond, followed by the cleavage of the C–O or P–O bond. The released sulphur was then oxidized.
To the knowledge of the authors, no systematic study of the effect of the substituent attached to the phenyl ring in triphenyl phosphorothionate or phosphate on the reactivity in oil solution has ever been carried out. The substitution of the phenyl ring with alkyl chains is usually made in order to enhance the solubility of the molecules in oil [28].
In the present work, the effect of the substituent bound to the phenyl ring of triphenyl phosphorothionates (t-butyl TPPT (b-TPPT) and p-nonyl TPPT (n-TPPT)) on the reactivity of these molecules in synthetic oil (poly-a-olefin) at high temperature (423 and 473 K) has been investigated for the first time by means of FT-IR and NMR spectros-copy. The surface chemistry of the 100Cr6 steel filings immersed in the heated oil solutions (473 K for 72 h) has been studied by XPS sputter depth profiling. A comparison with the results presented in our previous work and obtained using the unsubstituted molecule (i.e. TPPT) [21,
24] has also been performed.
2 Experimental 2.1 Materials
Poly-a-olefin (PAO, DurasynÒ166, Tunap Industrie GmbH & Co., Mississauga, Canada) was used as base oil and blended with t-butyl TPPT (b-TPPT, IrgalubeÒ 232, Ciba Speciality Chemicals, Basel, Switzerland) and p-nonyl TPPT (n-TPPT, IrgalubeÒ211, Ciba Speciality Chemicals, Basel, Switzerland) (Fig.1). The purification of the lubri-cant additives was carried out by liquid chromatography.
For the heating experiments, a 0.068 mol dm-3 (4.2 wt.%) solution of b-TPPT in PAO and a 0.043 mol dm-3 (3.77 wt.%) solution of n-TPPT in PAO were prepared. For dissolving the b-TPPT and the n-TPPT in the base oil, an ultrasonic bath was employed for four cycles of 10 min each. The temperature of the lubricant solution was always maintained below 318 K during the ultrasonic treatment.
In order to investigate the influence of metallic and oxidized iron/steel on the reactivity of alkylated TPPTs in oil solution, either 100Cr6 steel (AISI 52100) filings or metallic iron (Fluka, purum, reduced, C99.0% (RT), powder) or iron oxide particles were added to the lubricant oil solutions during the heating experiments. The iron oxide was prepared from (Fluka, purum, reduced, C99.0% (RT)) iron powder oxidized at 573 K for 3 h in Al2O3
crucibles (Metoxit AG, Thayngen, Switzerland)). In the following text, the metallic iron and iron oxide particles will be referred to as Fepartand FeOxpart, respectively. The
specific surface area of the 100Cr6 steel filings and of the metallic/oxidized iron particles was determined by N2
adsorption using the Brunauer, Emmett and Teller (BET) method and found to be 0.19 ± 0.01 m2g-1 and 0.07 ± 0.04 m2g-1, respectively [24]. The characterization of the surface chemistry of the as-received 100Cr6 steel filings, metallic iron and iron oxide particles by XPS is described in [24].
The vacuum filtration of the solutions at the end of the experiments was carried out using cellulose filter papers with 2.5 lm particle retention in liquid (Grade 42, What-manÒ, Maidstone, England).
2.2 Methods
2.2.1 Heating Experiments
The heating experiments were performed by immersing a glass flask (40 cm3, 95 9 27.5 mm2, VWR International, Dietikon, Switzerland) containing 15 cm3of the lubricant solution in a silicone oil bath that was heated at 423 K and at 473 K (±1 K). In order to investigate the influence of metallic and oxidized iron/steel on the reactivity of alkyl-ated TPPTs in oil solution, 1.25 ± 0.01 g of 100Cr6 steel Fig. 1 Chemical formula of alkylated triphenyl phosphorothionates (TPPTs). The alkyl chain R is t-butyl (b-TPPT) or p-nonyl (n-TPPT)
filings or Fepartor FeOxpartwas added to the oil solutions.
During the experiment the solution, open to air, was neither stirred nor purged with any gas. The duration of the heating varied between 3 and 168 h at 423 K and between 3 and 72 h at 473 K. The relative humidity was maintained between 22 and 30%.
After the test, the glass flask was removed from the silicone oil bath and left for 45 min in air to cool down without any cover. The solution was then vacuum filtered in order to remove any precipitates formed and the added 100Cr6 steel filings, Fepart and FeOxpart.
2.2.2 Microelemental Analysis
The microelemental analysis was carried out using a CHN-900 (Leco Corporation, Michigan, USA) for carbon and hydrogen, a CHNS-932 (Leco Corporation, Michigan, USA) for sulphur and a RO-478 (Leco Corporation, Michigan, USA) for oxygen. The phosphorus content was determined by the molybdovanadate method [29] after microwave digestion and leaching (2 9 50 min) in con-centrated sulphuric acid and perchloric acid at 483–508 K. 2.2.3 Fourier Transform Infrared Spectroscopy (FT-IR) Transmission FT-IR spectra were acquired with a NicoletTM 5700 Fourier Transform Infrared spectrometer (Thermo Electron Corporation, Madison, WI, USA). The experi-mental conditions are listed in Table1. Sampling was per-formed by placing one drop of solution onto a KBr pellet.
The spectra were processed with OMNICTM software (V7.2, Thermo Electron Corporation, Madison, WI, USA). A single-beam spectrum of the KBr pellet was acquired before each measurement as a background. Normalization was carried out with respect to the methyl asymmetric deformation band, overlapped by the methylene scissor vibration band of PAO at 1466 cm-1 [30].
2.2.4 Nuclear Magnetic Resonance (NMR) Spectroscopy The NMR spectra were recorded in CDCl3(99.8 at.% D,
Armar Chemicals, Do¨ttingen, Switzerland) at 300 K using a Bruker Avance 500 NMR spectrometer operating at
500.1 (1-H), 125.8 (13-C) and 202.5 (31-P) MHz. The chemical shifts, given as dimensionless values, were referred to TMS (1-H and 13-C) and H3PO4 (85%)
fol-lowing the IUPAC recommendation [31].
2.2.5 X-ray Photoelectron Spectroscopy (XPS)
The depth distribution of the elements present in the reaction layer formed on the 100Cr6 steel filings immersed in a 0.068 mol dm-3 solution of b-TPPT in PAO or in a 0.043 mol dm-3solution of n-TPPT in PAO heated at 473 K for 72 h was determined by XPS sputter depth profiling.
The XPS analyses were carried out using a PHI Quan-tera SXM (ULVAC-PHI, Chanhassen, MN, USA) spec-trometer. The X-ray source of the PHI Quantera SXM is a focused and scanned monochromatic Al Ka with beam diameters between 9 and 200 lm. The emitted electrons are collected and retarded with a Gauze lens system at an emission angle (EA) of 45°. After passing the hemispher-ical analyzer, the electrons are detected in a 32-channel detector. The system is equipped with a high-performance, floating-column ion gun and an electron neutralizer for charge control and correction. The points to be analysed were chosen using optical microscopic images in combi-nation with X-ray induced secondary electron images (SXI). The residual pressure in the spectrometer was always below 5 9 10-7Pa. The system was calibrated according to ISO 15472:2001 with an accuracy of ±0.1 eV.
During the sputter depth profile, data acquisition, which was performed using a beam size of 200 lm, with a power of 50 W in a constant-analyzer-energy (CAE) mode with a pass energy of 55 eV (full-width-at-half-maximum (FWHM) of the peak height for Ag 3d5/2 = 0.71 eV), was
alternated with Ar? ion sputtering (18 s at 3 kV, 3 9 3 mm2). The sputtering rate was measured on a Si/SiO2 reference sample under the same experimental
conditions and found to be 11.6 ± 0.1 nm/min (the standard deviation of the sputtering rate was calculated over three profiles acquired on the same sample). The sputtering depth profiles were processed using MultiPakTMsoftware (v8.1, ULVAC-PHI, Chanhassen, MN, USA). After applying an iterated Shirley-Sherwood background subtraction to the spectra, the apparent atomic in-depth concentration was calculated as: Xj¼ Iij Sij P jIij Sij ð1Þ
where Iijand Sijare the area and the sensitivity factor of the
peak i of the element j, respectively. The sensitivity factors were calculated from the Scofield photoionization cross-section [32], the angular asymmetry factor [33, 34], the Table 1 Transmission FT-IR experimental conditions
Detector DGTS/KBr Resolution 2 cm-1 Number of scans 64 Scan velocity 0.6329 cm/s Acquisition time 136 s Gain control 1
spectrometer transmission function and the inelastic mean free path (IMFP) corrected for the emission angle, assuming the sample to be homogenous.
The inelastic mean free path, i.e. the mean distance travelled by electrons with a given kinetic energy (KE) between inelastic collisions in a material M, corrected for the emission angle (H) was calculated as:
Ki;M¼ A KE2þ B ffiffiffiffiffiffiffi KE p cos hðnmÞ ð2Þ
where the values of A and B were 641 and 0.096, respec-tively. According to [35], these values are valid for inor-ganic compounds.
3 Results
3.1 Characterization of Purified b-TPPT and n-TPPT 3.1.1 Microelemental Analysis
The microelemental analyses of b-TPPT and n-TPPT after purification by liquid chromatography are reported in Table2 together with the expected weight percentages. The compositions were found to correspond to the molecular formulas C23H25O3PS and C45H69O3PS for
b-TPPT and n-TPPT, respectively. The results of the microelemental analysis suggest that the average degree of substitution (DS) per molecule of the phenyl rings of purified b-TPPT with t-butyl groups is one, while the average DS of the phenyl rings of purified n-TPPT with nonyl groups is three.
3.1.2 Fourier Transform Infrared Spectroscopy (FT-IR) The transmission FT-IR spectra of b-TPPT and n-TPPT after purification by liquid chromatography (Fig.2) clearly
Table 2 Elemental analysis of purified alkylated triphenyl phosphorothionates (t-butyl triphenyl phosphorothionate (b-TPPT) and p-nonyl triphenyl phosphorothionate (n-TPPT))
Element Concentration (wt.%)
b-TPPT n-TPPT
Measured Expected Measured Expected
Tri-substituted Bi-substituted Mono-substituted
C 66.46± 0.16 70.56 68.70 66.31 74.97± 0.15 74.96 H 6.05 ± 0.01 7.70 6.87 5.82 9.74± 0.2 9.65 O 11.97± 0.13 9.40 10.56 12.05 6.70± 0.14 6.66 P 7.68± 0.12 6.07 6.81 7.77 4.27 ± 0.01 4.30 S 7.84 ± 0.09 6.28 7.05 8.05 4.47± 0.1 4.45 Molecular formula C23H25O3PS C30H39O3PS C26H31O3PS C22H23O3PS C45H69O3PS C45H69O3PS
Fig. 2 Transmission FT-IR spectra of purified alkylated triphenyl phosphorothionates (TPPTs): a t-butyl triphenyl phosphorothionate (b-TPPT), b p-nonyl triphenyl phosphorothionate (n-TPPT). Three spectral regions can be distinguished: (i) 3150–2800 cm-1: aromatic CH stretching vibrations and alkane CH3, CH2stretching vibrations;
(ii) 2100–1650 cm-1: overtone and combination bands due to the aromatic CH out-of-plane deformation vibrations; (iii) 1600–400 cm-1 the fingerprint region
revealed the characteristic absorption bands of the b-TPPT and n-TPPT molecules. The IR peaks and the assigned functional groups are listed in Table3. Three spectral regions could be distinguished:
(i) the region at high wavenumbers (3150–2800 cm-1)— aromatic CH stretching vibrations and alkane CH3,
CH2stretching vibrations;
(ii) the region between 2100 and 1650 cm-1—overtone and combination bands due to the aromatic CH out-of-plane deformation vibrations; and
(iii) the fingerprint region (1600–400 cm-1).
All the reported vibration frequencies are in agreement with the literature [30,36–42].
3.1.3 Nuclear Magnetic Resonance (NMR) Spectroscopy The NMR spectra (1-H, 13-C and 31-P) of b-TPPT and n-TPPT after purification by liquid chromatography are shown in Fig.3.
Both the 1-H and 13-C NMR spectra of b-TPPT showed the characteristic signals of benzene in TPPT (not substi-tuted with any alkyl chain) [21,43,44]: in the 1-H spec-trum two doublets were found at 7.40 and 7.28 ppm, whereas in the 13-C spectrum four signals were detected at 150.9 (A), 129.9 (C), 125.7 (D) and 121.3 (B) ppm. The
partial substitution of the phenyl rings in TPPT with a t-butyl group resulted in the presence, in the 1-H spectrum, of a doublet at 7.19 ppm, assigned to protons in substituted benzene [43], and of a singlet at 1.35 ppm, which is characteristic of methyl protons in t-butyl [45]. In the 13-C spectrum, two additional peaks were found at 126.7 (D0) and 120.6 (B0) ppm, which were assigned to substituted benzene with a t-butyl group in para position [43–45]. Three signals were also detected between 30 and 35 ppm and corresponded to the methyl carbon atoms in t-butyl [43,44]. The 31-P NMR spectrum showed two multiplets at 54.4 and 51.4 ppm, whose integrated intensity ratio was 100:27. The first multiplet can be assigned to unsubstituted TPPT, the second one to mono-substituted b-TPPT [46–
48].
Both the 1-H and 13-C NMR spectra of n-TPPT showed the characteristic signals of para-substituted benzene [43,
45]: in the 1-H spectrum two peaks were found at 7.29 and 7.15 ppm, whereas in the 13-C spectrum three signals were detected at 148.4 (A), 127.8 (C) and 120.5 (B) ppm. The intense peaks detected between 0.2 and 2.0 ppm in the 1-H spectrum corresponded to the methyl and methylene pro-tons in the nonyl chain [43, 45]. The integrated intensity ratio of the peaks assigned to the para-substituted benzene and of those assigned to the nonyl chain was found to be 21:100, in agreement with the expected value. In the 13-C Table 3 IR frequencies (cm-1)
and functional groups for the transmission FT-IR spectra of purified alkylated triphenyl phosphorothionates (t-butyl triphenyl phosphorothionate (b-TPPT) and p-nonyl triphenyl phosphorothionate (n-TPPT))
m stretching, c out-of-plane deformation vibration, d in-plane deformation vibration
Frequency (cm-1) Functional group
b-TPPT n-TPPT
3096 w, 3061, 3043 3060, 3041w mCH aromatic 2963 s, 2905, 2869 2960 vs, 2930, 2872 m Alkane groups
1941,1887, 1862, 1780, 1724, 1652 1890, 1768, 1651 Overtone and combination bands due to cCH aromatic 1591, 1509, 1489 vs 1602, 1504 vs mPh 1456, 1442 1465 m dasCH3(overlapped with dCH2 in the case of n-TPPT) 1395, 1364 1408 vw, 1379 dsCH3 1331, 1307, 1288, 1268, 1220 1308, 1284, 1227 dCH aromatic 1188 vs, 1161 1203, 1169 vs mC–O–(P) 1110, 1087, 1070, 1054, 1024, 1016 1113, 1094, 1016 dCH aromatic 1008 mPh 943 b,vs 946, 934 vs mP–O–(C) 902, 846 cCH aromatic 837 m 838 m mC–C 798 s 799 w P=S (I) 771 m msP–O–(C) 754 699 w cCH aromatic 688 m 659 vw P=S (II) 655, 644, 616 dCH aromatic 559 570 dP–O–Ar 524, 498 546 cCH aromatic
spectrum, the characteristic signals of the nonyl chains between 0 and 45 ppm were very weak [43]. As for the 31-P NMR spectrum, a singlet was detected at 54.4 ppm, in agreement with the literature [46–48].
3.2 Heating Experiments
During the heating experiments at 423 K, the oil solutions slowly changed colour, turning yellow-brown after 13–24 h and dark brown after 72–168 h. At 473 K, the color change was faster: they turned yellow after 3 h and dark brown after 24 h.
After the experiments, the solutions were vacuum fil-tered in order to remove any precipitates formed and the
added 100Cr6 steel filings, Fepart and FeOxpart. No
oil-insoluble compounds were ever separated out on the filters.
3.2.1 Fourier-Transform Infrared Spectroscopy (FT-IR) 3.2.1.1 Heating at 423 K The transmission FT-IR spec-tra of a 0.068 mol dm-3solution of b-TPPT in PAO and of a 0.043 mol dm-3 solution of n-TPPT in PAO heated at 423 K for 3–168 h are reported in Fig.4.
The spectra of the unheated solutions (0 h) showed the characteristic peaks of b-TPPT and n-TPPT (Table3) and PAO (1466 cm-1 dasCH3 and dCH2, 1378 cm-1 dsCH3,
890 cm-1qCH3, 722 cm-1qCH2) [30,38,49] molecules.
Fig. 3 NMR spectra (1-H, 13-C and 31-P) of purified alkylated triphenyl phosphorothionates (TPPTs): a t-butyl triphenyl phosphorotionate (b-TPPT), b p-nonyl triphenyl phosphorothionate (n-TPPT)
The C–O–(P) stretching (1190 cm-1) and the P=S (I) (802 cm-1) vibration bands of b-TPPT in PAO shifted towards higher wavenumbers with respect to the trans-mission spectrum of purified b-TPPT (Table3). In the case of the solution of n-TPPT in PAO, no shift of the charac-teristic bands of the n-TPPT molecule was observed when compared to the transmission FT-IR spectrum of the purified compound (Table3).
Upon heating at 423 K, no changes in peak position were detected up to 168 h for both the solution of b-TPPT and n-TPPT in PAO. A slight decrease in absorbance of the characteristic vibration bands of b-TPPT (mPh at 1591 and 1489 cm-1, mC–O–(P) at 1190 cm-1, mP–O–(C) at 943 cm-1and P=S at 802 cm-1) and n-TPPT (mPh at 1602 and 1504 cm-1, mC–O–(P) at 1203 cm-1, mP–O–(C) at 946 cm-1and P=S at 799 cm-1) was observed after 168 h. A shoulder on the high-wavenumber side of the band assigned to the P–O–(C) stretching vibration was also detected after 168 h for both the solution of b-TPPT (arrow in Fig.4c) and n-TPPT (arrow in Fig.4d) in PAO.
In the 1850–1550 cm-1 region, where the C=O stretching vibration is found [30, 49], a broad but weak band with maximum at 1720 or 1722 cm-1 was detected after 168 h for the solution of b-TPPT or n-TPPT in PAO, respectively. This band exhibited a small peak at higher
wavenumbers (at 1771 cm-1or 1769 cm-1for b-TPPT or n-TPPT, respectively).
A slight increase of the baseline in the region between 1300 and 850 cm-1, where the C–O stretching vibration is found [30,49], was also observed after 168 h.
3.2.1.2 Heating at 473 K The transmission FT-IR spec-tra of a 0.068 mol dm-3solution of b-TPPT in PAO and of a 0.043 mol dm-3 solution of n-TPPT in PAO heated at 473 K are reported in Fig. 5. The duration of the heating was varied between 3 and 72 h.
A progressive decrease in absorbance of the character-istic vibration bands of b-TPPT (mPh at 1591 and 1489 cm-1, mC–O–(P) at 1190 cm-1, mP–O–(C) at 943 cm-1and P=S at 802 cm-1) and n-TPPT (mPh at 1602 and 1504 cm-1, mC–O–(P) at 1203 cm-1, mP–O–(C) at 946 cm-1and P=S at 799 cm-1) was detected upon heat-ing at 473 K. In comparison to the solution heated without any filing/particle, the presence of steel filings or Fepart or
FeOxpart in the oil solution during the heating experiments
turned out to induce a slower decrease in absorbance of the characteristic vibration bands of b-TPPT and n-TPPT.
A shoulder on the high-wavenumber side of the band assigned to the P–O–(C) stretching vibration was detected after 13 h for both the solution of b-TPPT and n-TPPT in Fig. 4 Transmission FT-IR
spectra of a 0.068 mol dm-3 solution of b-TPPT in PAO (a)/(c) and of a 0.043 mol dm-3 solution of n-TPPT in PAO (b)/(d) heated at 423 K for different times
PAO and found to increase in intensity with the heating time. In the case of the solution of b-TPPT, a well-defined peak at 959 cm-1 was detected after 72 h (arrow in Fig.5a, g). As for the solution of n-TPPT, the shoulder at 959 cm-1 turned into a well-defined peak after 24 h.
Increasing the heating time induced an increase in intensity of this band and a shift towards higher wavenumbers (961 cm-1 after 48 h and 963 cm-1after 72 h) (arrow in Fig.5b, h). In the case of the solutions (either with b-TPPT or with n-TPPT) heated with filings/particles, the shoulder Fig. 5 Transmission FT-IR spectra of a 0.068 mol dm-3solution of
b-TPPT in, PAO (a) and of a 0.043 mol dm-3solution of n-TPPT in PAO (b) heated at 473 K without any iron particle/steel filings, with 100Cr6 steel filings, metallic iron and iron oxide particles for 72 h.
The detailed mC=O, mC–O–(P) and mP–O–(C) regions are reported in c–h in the case of oil solutions heated at 473 K without any iron particle/steel filings for different times
on the mP–O–(C) band was less intense and no well-defined peak was ever detected (see Fig.5a, b).
Upon increasing the heating time at 473 K, a broad but weak band at 1241 cm-1was detected for both the solution of b-TPPT and n-TPPT in PAO (arrow in Fig.5a, b, e, f). The intensity of this new band was not influenced by the presence of steel filings/iron particles (Fepartor FeOxpart) in
the case of the solution containing b-TPPT. As for the oil solution with n-TPPT, a slightly more intense band was detected when the heating experiments were performed in the presence of 100Cr6 steel filings.
A progressive change of the shape of the band assigned to the C–O–(P) stretching vibration was also observed for the solution of b-TPPT heated at 473 K, when compared to the spectrum of the unheated solution (see Fig.5e). In the case of the n-TPPT solution heated at 473 K, a shoulder at higher wavenumbers characterized the mC–O–(P) absorp-tion band at 1169 cm-1(see Fig.5f). These changes could be due to the superposition of the C–O stretching vibration band at ca. 1175 cm-1as reported in [50].
In the 1850–1550 cm-1 region, where the C=O stretching vibration is found [30, 49], a broad band with maximum at 1721 cm-1 was detected in the case of the solution of b-TPPT heated with and without 100Cr6 steel filings and found to shift towards higher wavenumbers (1730 cm-1) after 24 h. No shift of the absorption band in the 1850–1550 cm-1 region, which had a maximum at 1721 cm-1, was detected when the heating experiments were carried out in the presence of Fepartor FeOxpart.
As for the solution of n-TPPT, the broad band at 1721 cm-1 (with a small peak at 1770 cm-1), which appeared in the transmission FT-IR spectra upon heating at 473 K for 3 h, did not undergo any shift when the heating experiments were performed in the presence of Fepart or
FeOxpart. For the solutions heated without any
filing/par-ticle and with 100Cr6 steel filings, a shift towards higher wavenumbers, i.e. 1730 cm-1, was found after 24 and 48 h, respectively.
3.2.1.3 Oxidation of the Base Oil In order to investigate the influence of alkylated TPPTs on the oxidation of the base oil, the area of the C=O stretching vibration band, after linear baseline subtraction, was plotted as a function of the heating time at 423 and 473 K (Fig.6). For com-parison purposes, control heating experiments on pure base oil (PAO) have been performed at the same temperatures investigated in this work, i.e. 423 and 473 K.
Compared to pure PAO, the presence of b-TPPT and n-TPPT in the solutions strongly reduced the oxidation of the base oil at 423 K (Fig.6a). The TPPT molecule substituted with nonyl chains was found to reduce the area of the carbonyl peak to a slightly higher extent than the
molecule with a t-butyl chain bound to the phenyl ring (inset of Fig. 6a).
At 473 K, both b-TPPT and n-TPPT did not reduce the area of the carbonyl peak as effectively as at 423 K. Moreover, the presence of either 100Cr6 steel filings, or Fepart or FeOxpart in the oil solution during the heating
experiments was found to slightly increase the oxidation of the base oil, when compared to the solutions heated with-out any filing/particles (Fig.6b, c).
3.2.1.4 Degradation Index (DI) In order to investigate the degradation kinetics, the degradation index (DI) has been defined as:
DI¼At;i A0;i
100 ð3Þ
where At,iis the absorbance of the i-th vibration at t heating
hours, while A0,iis the absorbance of the i-th vibration for
the unheated solution [21].
Figures7 and 8 report the DI of three characteristic vibrations of b-TPPT (mPh1591cm1;mP–O–(C), P=S (I)) and
four characteristic vibrations of n-TPPT (mPh1602cm1;
mCOðPÞ1203cm1; mP–O–(C), P=S (I)) for a 0.068
mol dm-3 solution of b-TPPT in PAO and a 0.043 mol dm-3solution of n-TPPT in PAO as a function of the heating time at 423 and 473 K.
The DI of the characteristic mC–O–(P) vibration of the b-TPPT molecule is not shown in Figs. 7 and8 since the superposition of the C–O stretching vibration appearing in the transmission FT-IR spectra as a result of the oxidation of the base oil resulted in DI values not representative of the decrease in absorbance of the mC–O–(P) band, as already noted in [21]. In the case of n-TPPT, since the C–O stretching vibration of the oxidation products of the base oil was found to be superimposed on the mC–O–(P) band at 1169 cm-1, the DI of mC–O–(P) band at 1203 cm-1 has been calculated and shown in Figs.7 and8.
At 423 K, all three absorption bands of b-TPPT showed similar values of the DI, which monotonically decreased with the heating time (logarithmic scale on the DI axis). In the case of the solution with n-TPPT, the DI of the four characteristic vibrations of this molecule decreased in the following order: mPh1602cm1 &mCOðPÞ1203cm1[
mP–O–(C) [ P=S (I). Comparing the DI of the P=S (I) absorption band for the two different additives, the lowest values were obtained in the case of the solution containing n-TPPT.
An increase in the degradation rate was observed for all absorption bands of b-TPPT and n-TPPT upon heating at 473 K.
In the case of the solution containing b-TPPT, all three bands showed a monotonic decrease of the DI (logarithmic
scale on the DI axis) independently of the presence of 100Cr6 steel filings, Fepart and FeOxpart. The DI values of
the three vibrations were comparable for the solutions heated in the presence of Fepart, whereas in the case of the
experiments carried out with steel filings or FeOxpart, the
P=S (I) band had a DI slightly lower than the other two peaks considered above. As for the solution heated without any filings/particles, the DI values of all three absorption
bands were lower than in the case of the solutions heated with filings/particles and decreased in the following order: mPh1591cm1[ mP–O–(C) [ P=S (I).
In the case of the solution with n-TPPT heated at 473 K without any filings/particles, the four characteristic absorption bands of n-TPPT showed a monotonic decrease of the DI (logarithmic scale on the DI axis) with the heating time. Moreover, the DI values decreased in the following Fig. 6 Area of the C=O
stretching vibration band as a function of the heating time at: a423 K for a 0.068 mol dm-3 solution of b-TPPT in PAO and for a 0.043 mol dm-3solution of n-TPPT in PAO; b 473 K for a 0.068 mol dm-3solution of b-TPPT in PAO; c 473 K for a 0.043 mol dm-3solution of
n-TPPT in PAO. The values of the area of the C=O stretching vibration band for pure PAO heated at the same temperatures are included for comparison
Fig. 7 Degradation Index (DI) versus heating time at 423 K for the characteristic vibrations of b-TPPT (a) and n-TPPT (b) in PAO
order: mPh1602cm1&mCOðPÞ1203cm1[ mP–O–(C) [
P=S (I). When the heating experiments were performed in the presence of steel filings or iron particles (Fepart or
FeOxpart), higher values of the DI were obtained. In
con-trast to the solution heated with Fepart, for which the DI of
the characteristic vibrations of n-TPPT had comparable Fig. 8 Degradation Index (DI)
versus heating time at 473 K for the characteristic vibrations of b-TPPT and n-TPPT in PAO
values, in the case of the experiments performed with steel filings or FeOxpart the P=S (I) band had slightly lower DI
values than the other vibrations considered above. 3.2.2 Nuclear Magnetic Resonance (NMR) Spectroscopy The NMR spectra (1-H, 13-C and 31-P) of 0.068 mol dm-3 solutions of b-TPPT and of 0.043 mol dm-3 solutions of n-TPPT in PAO heated at 423 and 473 K are shown in Fig.9
together with the spectra of the unheated solutions. In the 1-H and 13-C spectra, since the signals assigned to the t-butyl and nonyl chains bound to the phenyl ring of, respectively, the b-TPPT and n-TPPT molecules are superposed on those of PAO (i.e. between 0 and 2 ppm in the 1-H spectrum and between 10 and 45 ppm in the 13-C spectrum), only the regions where the benzene peaks are detected, are shown.
In the spectra of the unheated solutions, the character-istic peaks of b-TPPT and n-TPPT were found.
In the 1-H and 13-C NMR spectra of solutions of b-TPPT and n-TPPT in PAO heated at 423 and 473 K (with and without filings/particles), no new peaks appeared in the
region where the characteristic signals of benzene are found. New but weak peaks appeared in the 6–2 ppm region (not shown) of the 1-H spectra of the solutions heated at 473 K with and without steel filings and iron particles (Fepart or FeOxpart) and can be assigned to
hydrogen of a methyl or methylene group bound to a car-bonyl or ester group [45]. No signals that can be assigned to these species, can be found in the 13-C spectra due to the small magnetic moment and low natural abundance of carbon-13 [43,44].
In the 31-P spectrum of the unheated solution of b-TPPT (0 h), two multiplets at 54.5 and 51.5 ppm were detected, whose integrated intensity ratio was 100:29 ± 2. Upon heating at 423 K for 168 h, new but weak peaks were found at -15.7, -16.0, -16.3, -16.9 and -17.2 ppm (integrated intensity ratio of the multiplets at 54.5, 51.5 and of the signals between -15.5 and -17.5 ppm: I54.5 ppm:I51.5 ppm:
I-15.5/-17.5 ppm= 100:32 ± 1:6 ± 1). Upon increasing the
temperature to 473 K, the peaks appearing at higher field than the characteristic signals of b-TPPT increased in intensity: the integrated intensity ratio I54.5 ppm:I51.5 ppm:
Fig. 9 NMR spectra (1-H, 13-C and 31-P) of 0.068 mol dm-3 solutions of b-TPPT and of 0.043 mol dm-3solutions of n-TPPT in PAO: (1) unheated; (2) heated at 423 K for 168 h without any iron particle/steel filing; (3) heated at 473 K for 72 h without any iron particle/steel filing; (4) heated at 473 K for 72 h with 100Cr6 steel filings; (5) heated at 473 K for 72 h with metallic iron particles; and (6) heated at 473 K for 72 h with iron oxide particles
I-15.5 ppmwas found to be 100:39 ± 2:68 ± 4. When the
heating experiments were performed in the presence of steel filings or iron particles (Fepart or FeOxpart), the peaks
between -15.5 and -17.5 ppm were less intense. In the case of the solution heated with 100Cr6 steel filings, the inte-grated intensity ratio I54.5 ppm:I51.5 ppm:I-15.5 ppm was
100:39 ± 2:29 ± 3. As for the solution heated in the pres-ence of Fepart, a weak signal at -16.1 ppm was detected
(I54.5 ppm:I51.5 ppm:I-16.1 ppmwas 100:33 ± 1:8 ± 2), while
no new peaks appeared in the high-field region of the 31-P spectrum of the solution heated with FeOxpart.
In the case of the unheated solution of n-TPPT in PAO, a singlet at 54.6 ppm was detected in the 31-P spectrum. No new peaks were found in the spectrum of the solution heated at 423 K for 168 h. Upon heating at 473 K for 72 h with and without steel filings or iron particles (Fepart or FeOxpart), a
new signal was detected at -15.8 ppm. The intensity of this peak was lower when the heating experiments were per-formed in the presence of steel filings or iron particles (Fepart
or FeOxpart): in the case of the solution heated at 473 K
without any filing/particle the integrated intensity ratio of the peak at 54.6 and -15.8 ppm (I54.6 ppm:I-15.8 ppm) was
100:128 ± 3 and decreased to 100:26 ± 3 for the solution heated with 100Cr6 steel filings. In the case of the heating experiments carried out with Fepart and FeOxpart, the ratio
was 100:9 ± 1 and 100:11 ± 2, respectively. 3.2.3 XPS Sputtering Depth Profile
The depth distribution of the elements present in the reaction layers formed on the 100Cr6 steel filings immersed in the b-TPPT and n-TPPT solutions heated at 473 K for 72 h was determined by XPS sputter depth profiling. The apparent atomic concentration is reported in Fig.10as a function of the sputtering time.
On the filings’ surfaces only carbon, oxygen, phospho-rus and iron were detected (spectra not shown). Upon sputtering, the carbon concentration progressively decreased, indicating the removal of an organic layer. The oxygen profile had a maximum after 3.9 min (corre-sponding to a sputtering depth of 45.2 ± 0.4 nm relative to a Si/SiO2reference sample) and 3.0 min (corresponding to
a sputtering depth of 34.8 ± 0.3 nm relative to a Si/SiO2
reference sample) for the reaction layer formed in the presence of b-TPPT and n-TPPT, respectively. As for phosphorus, a peak at 134.4 ± 0.1 and 134.3 ± 0.1 eV (referencing the aliphatic carbon to 285.0 eV), values typical for phosphate/polyphosphate compounds [51, 52], was detected before the sputtering (0 min sputtering time) in the high-resolution (HR) spectra of the films generated from b-TPPT and n-TPPT, respectively. The phosphorus concentration increased with the sputtering time, reached a maximum after 3.0 min (corresponding to a sputtering depth of 34.8 ± 0.3 nm relative to a Si/SiO2 reference
sample) and 2.4 min (corresponding to a sputtering depth of 27.8 ± 0.2 nm relative to a Si/SiO2 reference sample)
for the layers formed from b-TPPT and n-TPPT, respec-tively, and went to zero after 6.6 min (corresponding to a sputtering depth of 76.6 ± 0.7 nm relative to a Si/SiO2
reference sample) of sputtering. No sulphur was ever detected in either reaction-layer sample during the sputter depth profiling.
4 Discussion
4.1 Antioxidant Property of Alkylated TPPTs
Lubricating oils are susceptible to thermo-oxidative ageing. The mechanism of hydrocarbon oxidation at high Fig. 10 X-ray photoelectron
spectroscopy (XPS) sputtering depth profile (Ar?ion sputtering at 3 kV, 3 9 3 mm2) of a 100Cr6 steel filing immersed in a 0.068 mol dm-3solution of b-TPPT in PAO (a) and in a 0.043 mol dm-3solution of
n-TPPT in PAO (b) heated at 473 K for 72 h
temperatures has been thoroughly investigated: it is driven by an autocatalytic process involving a four-step free-rad-ical reaction (initiation of the radfree-rad-ical chain reaction, chain propagation, chain branching and termination of the radical chain reaction), which leads to the formation of a complex mixture of oxygenated products, such as hydroperoxides, alkyl peroxides, alcohols, carboxylic acids, peroxy acids, esters, ketones, aldehydes, lactones, etc. [8,22,23].
As observed in our previous studies [21, 24], the appearance of a broad absorption band between 1850 and 1550 cm-1 and the increase of the baseline in the 1300–850 cm-1region in the FT-IR spectra, as well as the presence of new peaks between 6 and 2 ppm in the 1-H NMR spectra of solutions containing either b-TPPT or n-TPPT heated at 423 and 473 K with and without steel fil-ings or iron particles (Fepart or FeOxpart), clearly indicate
that the base oil (PAO) was getting oxidized during the heating process.
The presence of several overlapping peaks (corre-sponding to different carbonyl compounds [30,49,50,53]) contributing to the complex envelope at 1850–1550 cm-1 in the FT-IR spectra makes it difficult to identify the chemical nature and distribution of the species produced during the oxidation of the base oil. The shift of the peak maximum in the 1850–1550 cm-1 region of the FT-IR spectra of the solution of b-TPPT and n-TPPT heated at 473 K with and without 100Cr6 steel filings as the heating time increased, i.e. as the base oil got oxidized, might be due to the different distributions of the oxidation products, as already noted in [21,24]. Moreover, the shift towards lower wavenumbers of the carbonyl band when the heating experiments were carried out in the presence of Fepart or
FeOxpart, suggests that the oxidation reaction pathway of
the base oil is influenced by the presence of iron particles. The absorption band found in the FT-IR spectra of the solution of b-TPPT and n-TPPT heated at 423 and 473 K at ca. 1771 cm-1 can be assigned to five-membered-ring lactones and to small amounts of peroxy esters, while the peak at 1731–1720 cm-1is expected to correspond to ester compounds [30,49,50,53].
In the final stage, i.e. after heating the oil solutions at 423 K for 168 h and at 473 K for 72 h, the characteristic peaks of the b-TPPT and n-TPPT molecules were still detected in the FT-IR spectra. This finding, together with the appearance of the complex carbonyl absorption band between 1850 and 1550 cm-1, suggests that both the b-TPPT and n-TPPT molecules are not effective oxidation inhibitors, as opposed to ZnDTPs, which are known to act as both primary (radical trapping) and secondary (perox-ide-decomposing) antioxidants [6–9].
In order to gain an insight into the impact of b-TPPT and n-TPPT as well as into the effect of the added 100Cr6 steel filings, Fepartand FeOxparton the oxidation of the base oil, the
area of the carbonyl peak appearing between 1850 and 1550 cm-1in the FT-IR spectra was plotted as a function of the heating time. For comparison purposes, control heating experiments on pure PAO have been performed at the same temperatures investigated in this work, i.e. 423 and 473 K.
Compared to pure PAO, the presence of b-TPPT and n-TPPT in the oil solutions effectively reduced the area of the carbonyl peak at 423 K, indicating that the additives have a strong antioxidant effect at this temperature. The compari-son of these findings with the results of the FT-IR analysis of solutions of TPPT (without any alkyl chain attached to the phenyl ring) in PAO heated at 423 K, presented in our pre-vious work [21], allows the effect of the substituent in the TPPT molecules on the oxidation of the base oil to be investigated: the longer the alkyl chain bound to the phenyl rings, the larger the reduction of the area of the carbonyl peak at 423 K, i.e. the stronger the antioxidant effect.
At 473 K, neither b-TPPT nor n-TPPT was found to inhibit the oxidation of the base oil as effectively as at 423 K. Compared to TPPT [21], the presence of an alkyl chain (either t-butyl or p-nonyl) bound to the phenyl rings turned out to reduce the area of the carbonyl peak to a slightly higher extent when the solutions were heated with and without 100Cr6 steel filings, while no significant dif-ference was observed in the case of the experiments carried out with Fepart or FeOxpart.
The presence of transition metal ions having two valence states is known to lower the activation energy of the decomposition process of the alkyl hydroperoxide molecules formed during the autoxidation of hydrocarbons at high temperatures [8]. This explains the catalytic effect of steel filings and iron particles (Fepartor FeOxpart) on the
oxidation of the base oil observed in the case of the lubricant solutions containing b-TPPT and n-TPPT heated at 473 K. As for the solutions of TPPT heated at 473 K, the oxidation of the base oil was not catalyzed by the presence of steel filings or iron particles (Fepartor FeOxpart) [24].
4.2 Influence of the Alkyl Chain Length and Iron Oxidation State on the Reactivity of Alkylated TPPTs in Oil Solution
As was observed in the case of solutions of TPPT in PAO [21,24], the transmission FT-IR and NMR spectra showed the presence of the characteristic signals of the b-TPPT and n-TPPT molecules even after heating at 423 K for 168 h and at 473 K for 72 h. During the heating experiments, no oil-insoluble compounds were ever detected. These find-ings suggest that the alkylated TPPTs are highly thermally stable in oil solution, even in the presence of steel filings or iron particles (Fepart or FeOxpart). ZnDTP, in contrast,
is known to completely react at high temperature (403–503 K) within a few hours to produce a precipitate,
whose composition is similar to that of the films formed on tribological surfaces [6,10,54–56].
Although the oil solutions started to change color after 13–24 h at 423 K and after 3 h at 473 K, a shoulder on the high-wavenumber side of the mP–O–(C) peak was found only after heating for 168 h at 423 K and for 13 h at 473 K. A broad but weak band was also detected at 1241 cm-1for both the solution of b-TPPT and n-TPPT in PAO heated at 473 K, which can be assigned to the P=O stretching vibration [30]. The presence of this band sug-gests the exchange of the sulphur atom in the alkylated TPPTs molecules with a phosphoryl oxygen, as reported by Teichmann and Hilgetag [15] and as proposed in our pre-vious studies in the case of solutions of TPPT in PAO heated at 423 and 473 K with and without steel filings or iron particles (Fepartor FeOxpart) [21,24]. The presence of
an electron withdrawing oxygen bound to the phosphorus atom also explains the appearance of a shoulder at higher wavenumbers in the mP–O–(C) peak [42,57].
These band assignments could be confirmed by the NMR analyses. In the 31-P NMR spectra new peaks were found at higher fields (between -15.5 and -17.5 ppm) compared to the characteristic signals of the starting compounds, i.e. b-TPPT and n-TPPT, and corresponded to alkylated tri-phenyl phosphates (butylated tritri-phenyl phosphate (b-TPP) and nonylated triphenyl phosphate (n-TPP), respectively) [46,47]. The presence of the alkyl chain, either t-butyl in the case of b-TPP or p-nonyl in the case of n-TPP, bound to the phenyl rings is suggested by the 1-H spectra, which did not exhibit the appearance of any new peak in the region where the characteristic signals of benzene are found. In the case of the oil solutions containing b-TPPT, the detection of several peaks between -15.5 and -17.5 ppm in the 31-P NMR spectra could be due to the presence of different isomers of the b-TPP molecule [46].
As was the case for solutions of TPPT in PAO heated at 473 K [24], the signal at higher fields (between -15.5 and -17.5 ppm) in the 31-P NMR spectra decreased in intensity when either 100Cr6 steel filings, or Fepartor FeOxpartwere
added to the oil solutions containing b-TPPT and n-TPPT during the heating experiments. This finding, together with the corresponding decrease in intensity of the shoulder on the high-wavenumber side of the mP–O–(C) peak in the FT-IR spectra, suggests that the amount of alkylated triphenyl phosphates present in the lubricant was lower when the heating experiments were carried out in the presence of steel filings or iron particles (Fepartor FeOxpart). Such a lower
concentration of alkylated TPPs in the oil solution can be explained in two possible ways: either these molecules react on the surface of steel filings/iron particles and form a reaction layer, as indicated by the XPS sputter depth profile of the 100Cr6 steel filings immersed for 72 h in oil solutions heated at 473 K, and/or the oxygen present in the oil
preferentially takes part in the oxidation reaction of PAO (catalyzed by steel filings and iron particles (Fepart or
FeOxpart)) rather than in that of alkylated TPPTs. The
absence of any signal that can be assigned to intermediate species in the thermo-oxidative reaction of alkylated TPPTs, such as phosphites, for example, suggests that the reaction of alkylated TPPs on the filings/particles surface is the main reason for their lower concentration in oil solutions.
In our previous studies, a further insight into the deg-radation kinetics of TPPT in oil solution was provided by the DI [21,24]. The scission of the P=S bond as a starting point in the thermal reaction of TPPT was suggested by the DI values of the corresponding absorption band in the FT-IR spectra, which were always lower than the values of the Ph and P–O–(C) stretching vibration peaks. Moreover, the influence of the iron oxidation state on the degradation kinetics of TPPT could also be investigated: even if the reaction pathway turned out not to be affected by the presence of steel filings or iron particles (Fepartor FeOxpart)
in the lubricant solution, i.e. the scission of the thiophos-phoryl bond still started the decomposition reaction, the thermal degradation of TPPT was found to be catalyzed by the metallic iron particles.
In the present work, the DI of the characteristic vibration bands of the b-TPPT and n-TPPT molecules was calculated in order to have an insight into the reaction pathway as well as into the influence of temperature, alkyl chain length and iron oxidation state on the degradation kinetics.
As was the case for TPPT, the scission of the P=S bond turned out to be the starting point of the thermo-oxidative reaction of alkylated TPPTs at 423 and 473 K. Only in the case of the b-TPPT solution heated at 423 K, can a defin-itive statement about the reaction pathway not be made on the basis of the DI, since all the vibration bands considered had comparable DI values. As for the n-TPPT molecule, the superposition of the C–O stretching vibration of the oxidation products of the base oil upon the mC–O–(P) band at 1169 cm-1 allowed the DI of the mC–O–(P) band at 1203 cm-1to be calculated. As a consequence, the reaction step following the P=S bond scission could be identified on the basis of the FT-IR spectra: the cleavage of the P–O bond seems to preferentially occur over the breakage of the C–O bond.
The comparison of the DI of the P=S bond for solutions of b-TPPT and n-TPPT in PAO heated at 473 K with and without steel filings or iron particles (Fepart or FeOxpart)
also allowed the influence of the iron oxidation state on the degradation kinetics of alkylated TPPTs to be investigated. Compared to the solution heated without any filings/parti-cles, the presence of either 100Cr6 steel filings, or Fepartor
FeOxpart in the oil solution containing either b-TPPT or
n-TPPT during the heating process resulted in slightly higher DI values, indicating that the kinetics of the
thermo-oxidative reaction is slower. Moreover, while in the case of b-TPPT no significant difference in the DI of the P=S band was observed between the solutions heated with steel filings and iron particles (Fepart or FeOxpart), in the
case of the solutions containing n-TPPT the presence of Fepart during the heating process resulted in DI values
slightly higher than those calculated for the lubricants heated in the presence of 100Cr6 steel filings or FeOxpart.
These findings can be explained with the catalytic effect of steel filings and iron particles (Fepart or FeOxpart) on the
oxidation of the base oil, as indicated by the FT-IR results. As a consequence of the preferential reaction of the oxygen present in the oil solution with the base oil, the kinetics of the thermo-oxidative reaction of alkylated TPPTs is slower. A comparison with the data presented in our previous work can provide an insight into the effect of the length of the alkyl chain bound to the phenyl rings on the degrada-tion reacdegrada-tion [21,24]. At 423 K, the TPPT molecule (not substituted with any alkyl chain) turned out to be more reactive than alkylated TPPTs (with n-TPPT being more reactive than b-TPPT). At 473 K, the reactivity of TPPT and n-TPPT was found to be similar but higher than that of b-TPPT. In the case of the experiments carried out in the presence of steel filings or iron particles (Fepart or
FeOxpart), the reactivity of the TPPTs was demonstrated to
decrease with alkyl chain length.
Two effects might be taken into consideration for explaining this difference in the reactivity of TPPTs: electronic and solubility effects. As for the former, the presence of electron-releasing (ER) groups bound to the phenyl rings of arylphosphorothionates has been reported to increase the P=S bond length and the charge on the phosphorus atom [58]. This would make the phosphorus atom itself more open to nucleophilic attack, increasing, as a consequence, the reactivity of the molecule. Therefore, the electronic effect induced by the substitution of TPPT with alkyl chains does not explain the decrease in reactivity observed in the present paper.
On the other hand, the presence of an alkyl chain attached to the aromatic ring enhances the solubility of the molecule in lubricant oil, which would result in a lower amount of adsorbed additive at the steel (or iron)/oil interface. The different solubility in lubricant oil of the triphenyl phosphorothionate molecules employed in the present work and in our previous ones [21,24] can, then, explain the difference in the reactivity of these compounds in lubricant oil at high temperature.
4.3 Role of the Alkyl Chain in the Surface Chemistry of Alkylated TPPTs
A comparison of the results presented in this work with those outlined in our previous study [24] also allows the
role of the alkyl chain in the surface chemistry of alkylated TPPTs to be clarified. While in the case of TPPT (without any alkyl chain bound to the phenyl rings) the XPS sputter depth profile of 100Cr6 steel filings immersed for 72 h in oil solution heated at 473 K showed the presence of sul-phur starting from a sputtering depth of 3.5 ± 0.1 nm (relative to a Si/SiO2reference sample) [24], the reaction
layers formed from alkylated TPPTs (i.e. b-TPPT and n-TPPT) under the same experimental conditions did not contain any sulphur. This finding clearly suggests that the reaction mechanism strongly depends on the architecture of the molecule even within the same class of additives. In the following section a reaction mechanism for alkylated TPPTs will be proposed.
The fact that the reaction layers analysed in the present work differ from those investigated in the previous study carried out in our group with the same molecules can be ascribed to the more severe experimental conditions employed here: the heating temperature was 473 K (com-pared to 423 K in [26]) and the duration of the heating was extended to 72 h (compared to ca. 5 h in [26]). In partic-ular, Heuberger et al. [26], who studied the effect of the length of the alkyl chain on the chemistry of thermal films and tribofilms formed from alkylated TPPTs at 423 K on air-oxidized 100Cr6 steel discs, pointed out that both thermal films (non-contact region of the discs used for tribological tests) and tribofilms consist of a mixture of phosphates and sulphates. The composition turned out to depend on the chain length: with longer chains the phos-phate concentration decreased, while the sulphur concen-tration increased [26].
4.4 Proposed Reaction Mechanism
Based on the findings mentioned above, the reaction mechanism depicted in Scheme1is suggested.
As was already observed in the case of TPPT (without any alkyl chain bound to the phenyl rings) [21], at 423 and 473 K the temperature is high enough to start the decom-position of alkylated TPPTs in synthetic oil: the molecules undergo a nucleophilic attack (e.g. by means of OH groups) at the phosphorus atom to cause P=S bond scission to give alkylated triphenyl phosphite. Moreover, since according to the ‘‘Principle of Hard and Soft Acids and Basis’’ (HSAB), thiophosphoryl sulphur is a ‘‘soft base’’ and preferentially reacts with ‘‘soft acids’’, such as tetra-hedral carbon [16–19, 59], the alkylation of the sulphur atom can occur (through the isomerization of the molecule, which results in the migration of the alkyl group from the phenyl ring to the sulphur atom). The subsequent breakage of the weak P–S bond results in the formation of vola-tile compounds (i.e. thiols, where hydrogen is provided by the base oil) and alkylated triphenyl phosphite.
A thermo-oxidative reaction then takes place, which leads to the oxidation of alkylated triphenyl phosphite to alkyl-ated triphenyl phosphate.
In the case of b-TPPT, no statement can be made about the following reaction step, i.e. the cleavage of the P–O or C–O bond. In contrast, in the case of n-TPPT, since the C–O stretching vibration of the oxidation products of the base oil was found to be superimposed on the mC–O–(P) band at 1169 cm-1, the DI of mC–O–(P) band at 1203 cm-1could be calculated, allowing, as a consequence, the identification of the reaction step following the P=S bond scission: the P–O bond turned out to preferentially break over the C–O bond at both 423 and 473 K.
When the heating experiments are performed in the presence of steel filings, the alkylated triphenyl phosp-horothionate molecules might adsorb via the sulphur atom to the steel surface, in agreement with Heuberger et al. [25,
26] and as proposed in our previous study in the case of triphenyl phosphorothionate without any substituent attached to the phenyl rings [24]. A polar interaction between the OP–O–Ph atom and the nascent iron/steel
sur-face cannot be excluded, however, as pointed out by Koyama et al. [27].
The sulphur–metal coordination induces a partial posi-tive charge on the phosphorus atom, making the
phosphorus atom itself more open to nucleophilic attack [15]. The phosphoryl oxygen of alkylated triphenyl phos-phates, which are produced by the thermo-oxidative reac-tion of alkylated TPPTs in lubricant oil solureac-tion, can carry out a nucleophilic attack on the adsorbed phosphorothio-nate molecule, leading to the formation of pyro/polyphos-phates and sulphur-containing volatile compounds. This also results in a lower concentration of alkylated triphenyl phosphates in the oil solution, which corresponds well with the lower intensity of the signals assigned to these mole-cules in the FT-IR and NMR spectra of the solutions heated in the presence of steel filings or iron particles (Fepart or
FeOxpart) compared to the solutions heated without filings/
particles.
The decrease in intensity of the characteristic signals of alkylated TPPTs in the FT-IR spectra might not only be due to the chemical reaction of these compounds in oil solution. Ribeaud pointed out that the volatilization of these compounds is the main reason for the weight loss of triaryl monothiophosphates during thermogravimetric analyses (TGA) [60]. Even if different experimental con-ditions have been employed in the present work, some evaporation of alkylated TPPTs cannot be ruled out as a contributing factor in the observed decrease in absorbance of the characteristic vibration bands of alkylated TPPTs. Scheme 1 Reaction mechanism of alkylated triphenyl phosphorothionates (the alkyl chain R is t-butyl (b-TPPT) or p-nonyl (n-TPPT)) on 100Cr6 steel surface at 473 K
5 Conclusions
The following conclusions can be drawn from the results presented in this work:
(i) Alkylated TPPTs have a strong antioxidant effect on the base oil (poly-a-olefin, PAO) at 423 K. Increasing the length of the alkyl chain bound to the phenyl rings results in a stronger antioxidant effect at 423 K;
(ii) At 473 K, the alkylated TPPT molecules do not inhibit the oxidation of the base oil as effectively as at 423 K. The molecules substituted with an alkyl chain (b-TPPT and n-TPPT) reduce the oxidation of the base oil to a slightly higher extent than the molecule without any alkyl chain bound to the phenyl rings (TPPT);
(iii) The b-TPPT and n-TPPT molecules, being not completely decomposed upon heating at 423 K for 168 h and at 473 K for 72 h, have a high thermal stability even in the presence of metallic and oxidized iron/steel;
(iv) The thermo-oxidative reaction of alkylated TPPTs in oil solution starts with the scission of the P=S bond and leads to the formation of alkylated triphenyl phosphate. The reaction pathway is not dependent on the presence of steel filings/iron particles (Fepartor FeOxpart) in the lubricant;
(v) The concentration of alkylated triphenyl phosphate present in the oil solution and produced by the thermo-oxidative degradation of alkylated TPPTs is lower when the heating experiments are carried out in the presence of steel filings or iron particles (Fepartor FeOxpart);
(vi) The kinetics of the thermo-oxidative degradation of alkylated TPPTs is slower when steel filings or iron particles (Fepart or FeOxpart) are added to the oil
solution during the heating experiments;
(vii) At 423 K, the TPPT molecule, not substituted with any alkyl chain, is more reactive than alkylated TPPTs. At 473 K, the reactivity of TPPT and n-TPPT is comparable, but higher than that of b-TPPT. In the case of the experiments performed at 473 K in the presence of steel filings or iron particles (Fepart or FeOxpart), the reactivity of the
alkylated TPPT molecules decreases with the length of the alkyl chain bound to the phenyl rings; (viii) A reaction layer is formed on the 100Cr6 steel filings immersed in a solution of alkylated TPPTs for 72 h and found to consist of carbon, oxygen, phosphorus and iron. No sulphur was ever detected in the films during the sputter depth profiling.
Acknowledgements The authors wish to express their gratitude to the ETH Research Commission for its support of this work. Dr. H. Camenzind (Ciba Speciality Chemicals, Basel, Switzerland) is thanked for supplying the pure additive. Mrs. D. Sutter and Mr. M. Schneider kindly performed the NMR and the elemental analysis, respectively.
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